Various approaches have been developed to replenish combusting fluidized beds with solid fuel. Basically, the solid fuel is crushed and distributed through the bed while the bed is being violently agitated by fast rising bubbles of fluidizing air from forced draft fans. It is desired to supply fresh solid fuel with an arrangement to minimize elutriation of fines into the entraining gaseous products of combustion, obviate the formation of carbon monoxide, and absorb the sulfur compounds by limestone in the bed. Faced with the violence, or turbulance, with which the solid fuel is combusted during the upward flow of bubbles of fluidizing air, the introduction of fresh supplies of crushed solid fuel and makeup limestone is difficult to provide without aggravating the problems of elutriating fines, carbon monoxide formation, and sulfur escape.
The pneumatic transport of crushed solid fuel and limestone into the combusting fluidized bed is a wellknown system. The inherent disadvantage of this feed arrangement is its need for numerous and complex transport pipes whose discharge ends are stationed through the bed. The pneumatically transported solid material has a penchant for clogging their conducting pipes, creating a vicious maintenance problem. Additionally, transport air released at the feed nozzles in the bed, has a tendency to channel through the bed and thereby limit the residence time and lateral mixing of the fresh fuel within the bed with the result that sub-stoichiometric combustion is possible and elutriation of fines is enhanced.
Overbed feed systems for crushed, solid fuel and limestone have been developed. Without the agitation of transport air, many of the problems of elutriation and sub-stoichiometric combustion would appear to be mitigated. However, the descent of fresh supplies of solid material on the top of the combusting fluidized bed has large quantities of the fines elutriated into the freeboard above the bed without contact with the combustion temperature of the bed. The products of combustion, and partial combustion, are quickly cooled by the heat exchange surfaces above the bed freeboard, allowing the escape of carbon monoxide and sulfur compounds.
A third approach offers a higher potential for a satisfactory feed system. In this third approach, crushed solid fuel is accumulated into a slumped bed adjacent a combusting fluidized bed from which the fluidized bed is laterally fed, along with any necessary makeup limestone. The degree of turbulance, as an inherent characteristic of the combusting fluidized bed, provides the force for lateral distribution of the fresh fuel through the bed, once the fresh fuel flows into the bed. More specifically, the combusting fluidized bed can be regarded as boiling, in that bubbles of fluidizing air rise violently through the bed and force the solid crushed fuel laterally across the bed after the fuel has been fed into the side of the bed. Attractive as this side-feed system appears, the provision of a slumped bed at the side of the fluidized bed gives rise to problems in applying forces to consistently feed the fresh fuel from the slumped bed into the fluidized bed.